The present invention relates to a new component for switching an inductive load powered at a few kHz, e.g., a motor, a pump, a fan, etc., and more particularly to reducing the EMI and thus the size of an input filter.
Today in the automotive industry 80% of variable speed motor commands are made using a MOSFET drive in linear mode. This kind of drive dissipates a fraction of unused energy in the form of heat, about 110 W for a 450 W motor. 100 W dissipated in the form of heat represents 0.1 liters per 100 Km or 0.1 L/h gas consumption (2 miles per gallon). With the Kyoto Protocol, automotive vendors have to find a way to reduce the CO2 emission and improve the yield of a drive motor. For that reason, a 20 kHz switching drive is now being explored. 20 kHz is not audible to the human ear. This solution can reach up to 90% yield, but can create conduction electromagnetic perturbation (EMI) due to high frequency coupling. To reduce EMI of the input battery filter, the automotive vendors are forced to add a big input filter to a power switching circuit of the motor drive, thereby increasing the final price of the motor drive.
FIG. 1 illustrates the most common solution used to reduce EMI in a power switching circuit driving an inductive load. The power switching circuit includes high- and low-side transistors, e.g., MOSFETs, series connected at a switching node Vs, the switching node driving a load. FIG. 1 shows a source terminal of the high-side transistor Q1 coupled at the switching node Vs to a cathode of a diode Q2 and an inductor L1 (the load) connected between the switching node Vs and ground. In accordance with that solution, resistor Rg is placed in a path of a current Igate providing PWM pulses to a gate terminal of the transistor Q1. The placement of resistor Rg at the gate terminal allows smooth injection of the current in the gate terminal. As shown in FIG. 2, which illustrates gate to source and drain to source voltage and drain to source current when the transistor Q1 is ON and OFF, the drawback of this solution is that the switching losses are maximal, therefore the transistor Q1 heats up a lot.
Another way to reduce the EMI, as illustrated in FIG. 3, was explored a few years ago. The main idea behind this method is to perform gate shaping, i.e., changing the gate profile over time. In particular, FIG. 3 illustrates, in steps 1 and 2 pre-charging of a gate terminal of the transistor Q1 to voltage Vth and to gate to source voltage ON Vgson at step 4 and the effect on load current I.sub.load through the load L1 and the drain to source voltage Vds. This solution takes a huge step forward proving that the gate shaping works, and if it is well set up can reduce the EMI. As voltage Vth value is not known in advance, a complicated loop must be developed to auto-adapt the circuit. After several cycles, pre-charge voltage Vprech may reduce and regulate the turn ON delay with an external consign.
However, applying the same principle for the turn OFF becomes complicated because the source voltage moves very quickly and it becomes very difficult to catch the right event at the right moment. Due to these difficulties an improved, more robust, and easier to manufacture system is necessary.